When network testing, it is always important to behave responsibly and ethically. Generally speaking, don't test someone else's network without their explicit permission. Doing otherwise could cause embarrassment at best, and land you in serious legal trouble at worst.
You should also be aware that some network testing techniques can set off alarms. For example, many network administrators monitor their network's load and performance characteristics. If you decide to load-test these networks without notice, you might set off automated alarms causing inconvenience.
Some other testing techniques can look like attacks. Port scanning, for example, is a useful technique where a tester tries establishing many connections on different ports. It's used to discover which ports on a system are open. However, it is a common technique used by...
